Please Help Me Answer This Point to Point Leased Line Vs Frame Relay

[mnoote191]

Hi My name is Michael in I am having problems understanding Point-to-Point Leased Lines and Frame Relay I understand that there are 3 encapsulation protocols Frame Relay, Point to Point Protocol, and HDLC by Default. According to the Cisco Book I am reading I am trying to get my CCENT and it says that majority of the time Point - to -Point Leased Lines use (PPP) and (HDLC) as there default encapsulations. I looked at a Diagram with Point-to Point Leased Lines it shows a Router on one end connected to a CSU/DSU then connected to a WAN Switch at the Telephone Company End and then on the other side of the WAN it shows the other router connected to a CSU/DSU then conected to a WAN Switch and in the book it states for Point to Point Leased Line connection with a 100 sites it would require every router to have 99 serial interfaces and basically that all would be connected to each other from site to site. Now with Frame-Relay it shows a router connected to a Frame Relay Switch on one side of Telephone Companys Wan Cloud  and shows the other router at the remote site connected to another Frame Relay switch on the other side of Telephone Companys part of the Wan Cloud that frame relay uses to DLCI Permanent Virtual Circuits basicallys DLCI are associated with Virtual Circuits.What I cant understand is why when both diagrams show a Point-to-Point Leased Line where both sites are connected to WAN Switches which are then connected together and on Frame Relay diagram shows both sites connected to Frame Relay Switches why would Point to Point require all those extra cables and not Frame Relay when they both go through the Telephone Company Service Providers switches which then in return connect to each other this is what I see out of the Cisco Book I can figure it out                 

Thanks

[Peter Paluch]

Hi Michael,

why would Point to Point require all those extra cables and not Frame  Relay when they both go through the Telephone Company Service Providers  switches

Think of this: If you are using PPP then this protocol is capable of talking to only a single device on a link - to the opposite router. There is no address information in the PPP frame header to explain which router it is addressed to and from. If a router could speak to several neighboring routers at a time through the same serial interface, there would be no way to explain who sent the PPP frame and for whom it is destined. Therefore, if using PPP, a single serial interface must be connected only to a single neighboring router (forget about the CSU/DSU, WAN switches and other service provider devices inbetween - they do not process your PPP frame as data, just as signals). With PPP, you are always talking a router to a single neighboring router, with the PPP session being terminated on both routers. The provider's devices do not process the PPP frames, they just carry the signals produced by the CSU/DSU.

With Frame Relay, there is an addressing field (the DLCI) present in the frame header and the provider's devices will be processing the frame in a hop-by-hop fashion as a datagram. They will use the DLCI to forward the frame along the PVC. With the addressing information and the service provider devices actually knowing where to send the frame next, you can substitute multiple physical interfaces with multiple virtual circuits - because the frame header allows addressing multiple PVCs (thereby multiple neighbors), and the devices in the SP cloud will intelligently forward the frame according to the address field.

I am not sure if this helps so please feel welcome to ask further.

Best regards,

Peter

[Richard Burts]

Michael

I like the explanation from Peter. There is another way to look at your question and perhaps this might help as a supplement to Peter's answer.

When you use PPP or HDLC you are using a dedicated circuit end to end. Only your traffic uses that circuit, you get 100% of the bandwidth of that circuit, and you pay for 100% of the circuit. Frame Relay was designed as a shared service. With Frame Relay you share the circuit with other customers, and you pay for only part of the cost of the circuit. Part of what makes Frame Relay work is that the provider configures multiple virtual circuits that are carried on the one physical circuit.

So the example in the book is correct (if a bit extreme). If your site has 99 other sites that it needs to communicate with (and if you want a direct connection between every site and every other site) then each site would need 99 circuits to connect to 99 other sites using a dedicated circuit for each site. But since Frame Relay has multiple virtual circuits on the physical circuit then you could have 99 virtual circuits on your Frame Relay connection and could connect to 99 neighbors using only the single serial line for Frame Relay.

HTH

Rick

[mnoote191]

Thank you very much for your help Richard

[lmarkert1]

Just a note, Michael this is just the explanation I was looking for. It just unlocked the puzzle box for me. Thanks much

[mnoote191]

I greatly appreciate your help peter thanks so much

[mnoote191]

One question with frame relay you are connected with a physical cable to the frame relay switches and how do the frame relay switches connect at the telephone company end are those all connected with a physical cable as well but they are virtual circuits created within that physical connection between the frame relay circuits

With Point to Point leased lines even though you need a physical connection from each router to each router is just that the switches are not Frame Relay, but WAN Switches as the book calls them that the WAN switches dont know how to forward the data so there your routers are basically forwarding the traffic to one another I am assuming using like a static route of some sort and that with Frame Relay switches and Frame Relay the switches know how to route the traffic because of DLCI along with the fact that Frame Relay switches are smarter for lack of better terms than the regular WAN Switches that Point to Point use

You have to forgive I am new to all of this I failed the CCENT one time dont want to fail it again thanks for all your help

[Peter Paluch]

Hi Michael,

with frame relay you are connected with a physical cable to the frame  relay switches and how do the frame relay switches connect at the  telephone company end are those all connected with a physical cable as  well but they are virtual circuits created within that physical  connection between the frame relay circuits 

The Frame Relay switches may be interconnected with Serial interfaces similar to routers, or they may use different technologies to carry their frames as long as the the frames are passed transparently without modification. Usually, when speaking about Frame Relay, the discussion focuses on the interface between the customer and the first FR switch, while the switch interconnections are not usually commented upon.

With Point to Point leased lines even though you need a physical  connection from each router to each router is just that the switches are  not Frame Relay, but WAN Switches as the book calls them that the WAN  switches dont know how to forward the data

Yes... you see, the WAN switches are simply devices that amplify your signal and carry it over large distances. They do not carry about the information carried by that signal. It is possible that these devices digitize the signal and carry it in digital form but even if they do that, they do not process the information. For all practical purposes, the circuit running over several WAN switches can be seen as a long uninterrupted wire on which the signal does not significantly degrade. The bit one router puts on this point-to-point is the bit the other router will receive. There is no processing of these bits inbetween, apart from regenerating and amplifying the signal.

You have to forgive I am new to all of this I failed the CCENT one time dont want to fail it again thanks for all your help

You are very much welcome. No apologies are necessary.

Best regards,

Peter